The invention relates to radio frequency power amplifiers, and more particularly to such power amplifiers utilizing field-effect-transistors (FET's).
An important goal in the design of radio frequency (RF) power amplifiers used for high performance data transmission is low intermodulation distortion (IMD), which is caused by non-linearities in the amplifying device. In a FET amplifier operated in the Class AB mode, the distortion is increased by operation of the device in the cutoff region.
In the prior art intermodulation distortion was reduced by providing negative feedback between the FET source and gate. However, in order to obtain the proper phase relationship in the feedback signal, a network of reactive components was conventionally used. Using this technique the distortion components are typically 30 db below the desired signal when a two-tone test is used, which exceed the typically desired level of 35 db. However, the use of such components restricts the gain of the amplifier to a relatively narrow bandwidth, making the amplifier inappropriate for the broadband designs which are widely used in modern equipment. Levels below 45 db can be achieved without the use of reactive feedback components using the feedforward technique, but this technique is complex and expensive.
It is therefore an object of the present invention to provide an RF amplifier having reduced intermodulation distortion.
It is another object of the present invention to provide an RF amplifier which is broadbanded.
It is a further object of the present invention to provide an RF amplifier with the foregoing characteristics which non-complex and less expensive prior art techniques.